Dc-dc converter

ABSTRACT

A DC-DC converter including a Pulse Width Modulation (PWM) controller for converting an input voltage into an output voltage is provided. The PWM controller includes an error amplifier, a comparator, a PWM generator and a ramp generator. The error amplifier generates an error signal according to a difference between a reference voltage and the output voltage. The comparator compares the error signal with a ramp signal to generate a trigger signal. The PWM generator generates a PWM signal with a fixed turn-on time, wherein a frequency of the PWM signal is adjusted according to the trigger signal, the input and output voltages. The ramp generator generates the ramp signal according to the PWM signal, the input voltage and the output voltage.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a controller of a DC-DC converter, and more particularly to a DC-DC converter with a constant on time (COT) pulse width modulation (PWM) controller.

2. Description of the Related Art

DC-DC converters are widely used for various electronic devices. A constant on-time (COT) voltage regulator is one type of DC-DC converter. In general, a COT voltage regulator may turn on a main switch during a fixed period when a feedback voltage is smaller than a reference voltage, and the COT voltage regulator may adjust a turn off period of the main switch so that a steady output voltage may be provided. An output capacitor with a high equivalent series resistance (ESR) disposed in parallel with a load is necessary for a conventional COT voltage regulator, so that a steady output voltage may be provided. However, although a high ESR may help to provide system stability, for a COT voltage regulator, probability of output ripples increase due to the high ESR, which negatively influence the output voltage and power conversion efficiency of the COT voltage regulator.

U.S. Pat. No. 6,583,610 discloses a voltage regulator which operates in ripple-mode and comprises a virtual ripple generator. The virtual ripple generator provides a regulator feedback signal that includes a generated ripple component as a composite signal. The composite signal is generated according to an actual output signal and a ripple signal synchronized to switching cycles of the voltage regulator. Thus, the regulation feedback signal reflects the DC value of the output signal and is responsive to transient changes in the output signal level.

BRIEF SUMMARY OF THE INVENTION

DC-DC converters are provided. An exemplary embodiment of the DC-DC converter for converting an input voltage into an output voltage is provided. The DC-DC converter comprises an input node for receiving the input voltage, an output node for providing the output voltage to a load, an inductor coupled between the output node and a first node, a first transistor coupled between the input node and the first node, a second transistor coupled between the first node and a ground, and a Pulse Width Modulation (PWM) controller. The PWM controller comprises an error amplifier, a first comparator, a PWM generator, and a ramp generator. The error amplifier receives a reference voltage and the output voltage to generate an error signal according to a difference between the reference voltage and the output voltage. The first comparator compares the error signal with a ramp signal to generate a trigger signal. The PWM generator generates a PWM signal with a fixed turn-on time, wherein a frequency of the PWM signal is adjusted according to the trigger signal, the input voltage and the output voltage. The ramp generator generates the ramp signal according to the PWM signal, the input voltage and the output voltage. The PWM controller provides the PWM signal to control the first transistor and the second transistor, so as to convert the input voltage into the output voltage.

Furthermore, another exemplary embodiment of a DC-DC converter for converting an input voltage into an output voltage is provided. The DC-DC converter comprises an input node for receiving the input voltage, an output node for providing the output voltage to a load, an inductor coupled between the output node and a first node, a first transistor coupled between the input node and the first node, a second transistor coupled between the first node and a ground, and a PWM controller. The PWM controller comprises an error amplifier, a sense unit, a compensation unit, a first comparator, a PWM generator, and a ramp generator. The error amplifier receives a reference voltage and the output voltage to generate an error signal according to a difference between the reference voltage and the output voltage. The sense unit senses the inductor to generate a sense current. The compensation unit generates a compensation signal according to the error signal and the sense current. The first comparator compares the compensation signal with a ramp signal to generate a trigger signal. The PWM generator generates a PWM signal with a fixed turn-on time, wherein a frequency of the PWM signal is adjusted according to the trigger signal, the input voltage and the output voltage. The ramp generator generates the ramp signal according to the PWM signal, the input voltage and the output voltage. The PWM controller provides the PWM signal to control the first transistor and the second transistor, so as to convert the input voltage into the output voltage.

Moreover, another exemplary embodiment of a DC-DC converter for converting an input voltage into an output voltage is provided. The DC-DC converter comprises an input node for receiving the input voltage, an output node for providing the output voltage to a load, an inductor coupled between the output node and a first node, a first transistor coupled between the input node and the first node, a second transistor coupled between the first node and a ground, and a PWM controller. The PWM controller comprises an error amplifier, a sense unit, a compensation unit, a first comparator, a PWM generator, and a ramp generator. The error amplifier receives a reference voltage and the output voltage to generate an error signal according to a difference between the reference voltage and the output voltage. The sense unit generates a sense current corresponding to a loading of the load. The compensation unit generates a compensation signal according to the sense current and a ramp signal. The first comparator compares the compensation signal with error signal to generate a trigger signal. The PWM generator generates a PWM signal with a fixed turn-on time, wherein a frequency of the PWM signal is adjusted according to the trigger signal, the input voltage and the output voltage. The ramp generator generates the ramp signal according to the PWM signal, the input voltage and the output voltage. The PWM controller provides the PWM signal to control the first transistor and the second transistor, so as to convert the input voltage into the output voltage.

A detailed description is given in the following embodiments with reference to the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein:

FIG. 1 shows a DC-DC converter according to an embodiment of the invention;

FIG. 2 shows a waveform diagram illustrating the relationship between the PWM signal S_(PWM) and a current IL flowing through the inductor L of FIG. 1;

FIG. 3A shows a PWM generator according to an embodiment of the invention;

FIG. 3B shows a waveform diagram of the signals in the PWM generator of FIG. 3A;

FIG. 4A shows a ramp generator according to an embodiment of the invention;

FIG. 4B shows a waveform diagram of the signals in the ramp generator of FIG. 4A;

FIG. 5 shows an example illustrating a waveform diagram of the signals of the DC-DC converter of FIG. 1;

FIG. 6 shows another example illustrating a waveform diagram of the signals of the DC-DC converter of FIG. 1;

FIG. 7 shows a DC-DC converter according to another embodiment of the invention.

FIG. 8 shows a DC-DC converter according to another embodiment of the invention;

FIG. 9 shows a DC-DC converter according to another embodiment of the invention; and

FIG. 10 shows a DC-DC converter according to another embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims.

FIG. 1 shows a DC-DC converter 100 according to an embodiment of the invention. The DC-DC converter 100 converts an input voltage V_(IN) received from an input node N_(in) into an output voltage V_(OUT). The DC-DC converter 100 comprises two transistors MU and ML, an inductor L, a control unit 110 and a PWM controller 120. The transistor MU is coupled between the input node N_(in) and a node N₁, and the transistor ML is coupled between the node N₁ and a ground GND. In this embodiment, the transistors MU and ML are N-type transistors which function as the switches. The control unit 110 receives a pulse width modulation (PWM) signal S_(PWM) provided by the PWM controller 120 and controls the transistors MU and ML to switch on or off according to the PWM signal S_(PWM). The inductor L is coupled between the node N₁ and an output node N_(out), wherein the output voltage V_(OUT) is outputted to a load 180 via the output node N_(out). Furthermore, an output capacitor C1 with a lower equivalent series resistance (ESR) is coupled between the N_(out) and the ground GND, and a resistor Resr represents an ESR of the output capacitor C1.

As shown in FIG. 1, the PWM controller 120 comprises a ramp generator 130, a PWM generator 140, a compensation unit 150, an error amplifier 160 and a comparator 170. The error amplifier 160 receives a reference voltage V_(REF) and the output voltage V_(OUT) to generate an error signal V_(ERR) according to a difference between the reference voltage V_(REF) and the output voltage V_(OUT). The compensation unit 150 coupled between an output terminal of the error amplifier 160 and the comparator 170 is used to compensate the error signal V_(ERR), and the compensation unit 150 comprises a resistor 152 coupled to the output terminal of the error amplifier 160 and a capacitor 154 coupled between the resistor 152 and the ground GND. After the error signal V_(ERR) is compensated, the comparator 170 compares the error signal V_(ERR) with a ramp signal S_(RAMP) provided by the ramp generator 130 to generate a trigger signal S_(TR). The PWM generator 140 generates the PWM signal S_(PWM) according to the trigger signal S_(TR), the input voltage V_(IN) and the output voltage V_(OUT). The ramp generator 130 generates the ramp signal S_(RAMP) according to the PWM signal S_(PWM), the input voltage V_(IN) and the output voltage V_(OUT).

FIG. 2 shows a waveform diagram illustrating the relationship between the PWM signal S_(PWM) and a current I_(L) flowing through the inductor L of FIG. 1. Referring to FIG. 1 and FIG. 2 together, during a period T_(on) (i.e. a turn-on time of the PWM signal), the PWM signal S_(PWM) controls the transistor MU to turn on and controls the transistor ML to turn off. During a period T_(off) (i.e. a turn-off time of the PWM signal), the PWM signal S_(PWM) controls the transistor MU to turn off and controls the transistor ML to turn on. As shown in FIG. 2, the current I_(L) has a minimum current value I_(min) (ex. I_(min)=0) at time t1, and then the current I_(L) starts to increase and reaches a maximum current value I_(max) at time t2, wherein I_(max)=2×I_(avg) and I_(avg) represents an average current value of the current I_(L). Next, the current I_(L) starts to decrease and reaches the minimum current value I_(min) at time t3. A rising slope S1 of the current I_(L) may be given by the following Equation (1):

$\begin{matrix} {{S\; 1} = {\frac{V_{IN} - V_{OUT}}{L} = {\frac{2 \times I_{avg}}{T_{on}}.}}} & (1) \end{matrix}$

According to the Equation (1), the period T_(on) may be given by the following Equation (2):

$\begin{matrix} {T_{on} = {\frac{2 \times I_{avg} \times L}{V_{IN} - V_{OUT}}.}} & (2) \end{matrix}$

In addition, a falling slope S2 of the current I_(L) may be given by the following Equation (3):

$\begin{matrix} {{{S\; 2}} = {\frac{V_{OUT}}{L} = {\frac{2 \times I_{avg}}{T_{off}}.}}} & (3) \end{matrix}$

According to the Equation (3), the period T_(off) may be given by the following Equation (4):

$\begin{matrix} {T_{off} = {\frac{2 \times I_{avg} \times L}{V_{OUT}}.}} & (4) \end{matrix}$

Therefore, according to the Equations (2) and (4), a period T and a frequency F_(sw) of the PWM signal S_(PWM) may be given by the following Equations (5) and (6), respectively:

$\begin{matrix} {{T = {{T_{on} + T_{off}} = {2 \times I_{avg} \times {L\left( {\frac{1}{V_{IN} - V_{OUT}} + \frac{1}{V_{OUT}}} \right)}}}};{and}} & (5) \\ {F_{sw} = {\frac{1}{T} = {\frac{1}{2 \times I_{avg} \times L} \times {\frac{\left( {V_{IN} - V_{OUT}} \right) \times V_{OUT}}{V_{IN}}.}}}} & (6) \end{matrix}$

Suppose that the period T_(on) has a relationship with a ratio of the output voltage V_(OUT) to the input voltage V_(IN), i.e.

${T_{on} = {{RCK}\left( \frac{V_{OUT}}{V_{IN}} \right)}},$

wherein the parameters R, C and K are constant. Therefore, the period T_(on) may be rewritten as the following Equation (7) to obtain the following Equation (8):

$\begin{matrix} {{T_{on} = {{{RCK}\left( \frac{V_{OUT}}{V_{IN}} \right)} = \frac{2 \times I_{avg} \times L}{V_{IN} - V_{OUT}}}};{and}} & (7) \\ {{2 \times I_{avg} \times L} = {{{RCK}\left( \frac{V_{OUT}}{V_{IN}} \right)}{\left( {V_{IN} - V_{OUT}} \right).}}} & (8) \end{matrix}$

According to the Equation (8), the period T_(off) may be rewritten as the following Equation (9):

$\begin{matrix} {T_{off} = {\frac{2 \times I_{avg} \times L}{V_{OUT}} = {{{RCK}\left( \frac{V_{IN} - V_{OUT}}{V_{IN}} \right)}.}}} & (9) \end{matrix}$

Thus, according to the Equations (7) and (9), the period T of the PWM signal S_(PWM) may be rewritten as the following Equation (10):

$\begin{matrix} \begin{matrix} {T = {T_{on} + T_{off}}} \\ {= {{{RCK}\left( \frac{V_{OUT}}{V_{IN}} \right)} + {{RCK}\left( \frac{V_{IN} - V_{OUT}}{V_{IN}} \right)}}} \\ {= {{RCK}.}} \end{matrix} & (10) \end{matrix}$

Due to the parameters R, C and K being constant, the period T of the PWM signal S_(PWM) is fixed.

FIG. 3A shows a PWM generator 300 according to an embodiment of the invention. The PWM generator 300 comprises an amplifier 310, a current generating unit 320, a comparator 330, a transistor M1, a resistor R_(RT) and a capacitor C_(ON). The amplifier 310 has an inverting input terminal coupled to a node N₂, a non-inverting input terminal for receiving a voltage V1 and an output terminal coupled to a gate of the transistor M1, wherein the voltage V1 is a voltage in proportion to the input voltage V_(IN), i.e. V1=K1×V_(IN). The current generating unit 320 is used as an example for description, and does not limit the invention. For example, the current generating unit 320 may be a current mirror circuit. When the trigger signal S_(TR) is triggered, a current I1 provided by the current generating unit 320 may flow through the transistor M1 and the resistor R_(RT), wherein a current value of the current I1 is determined according to the voltage V1 and the resistor R_(RT), ex. I1=V1/R_(RT)=K1×V_(IN)/R_(RT). Simultaneously, the capacitor C_(ON) is charged by a current I2 provided by the current generating unit 320 when the trigger signal S_(TR) is triggered. In one embodiment, a current value of the current I2 is equal to that of the current I1, ex. I2=K1×V_(IN)/R_(RT). In another embodiment, the current I2 is a current in proportion to the current I1.

FIG. 3B shows a waveform diagram of the signals in the PWM generator 300 of FIG. 3A. Referring to FIG. 3A and FIG. 3B together, a voltage V_(C) represents a voltage across the capacitor C_(ON). The comparator 330 is used to compare the voltage V_(C) with a voltage V2, wherein the voltage V2 is a voltage in proportion to the output voltage V_(OUT), i.e. V2=K2>V_(OUT). When the voltage V_(C) is smaller than the voltage V2, an active state of the PWM signal S_(PWM) is asserted, i.e. the period T_(on). On the contrary, when the voltage V_(C) is larger than the voltage V2, an inactive state of the PWM signal S_(PWM) is asserted, i.e. the period T_(off). Therefore, the period T_(on) and the period T_(off) may be given by the following Equations (11) and (12), respectively:

$\begin{matrix} {{T_{on} = {{\frac{C_{ON}}{I\; 2}{dV}\; 2} = {R_{RT}{C_{ON}\left( \frac{K\; 2}{K\; 1} \right)}\frac{V_{OUT}}{V_{IN}}}}};{and}} & (11) \\ {T_{off} = {R_{RT}{C_{ON}\left( \frac{K\; 2}{K\; 1} \right)}\frac{V_{IN} - V_{OUT}}{V_{IN}}}} & (12) \end{matrix}$

Due to the resistor R_(RT), the capacitor CON and the parameters K1 and K2 being constant, the period T_(on) and the period T_(off) are determined according to the input voltage V_(IN) and the output voltage V_(OUT).

FIG. 4A shows a ramp generator 400 according to an embodiment of the invention. The ramp generator 400 comprises an amplifier 410, two transistors M2 and M3, a capacitor C_(OFF) and a current source 420. The amplifier 410 has an inverting input terminal coupled to the transistor M2, a non-inverting input terminal for receiving a voltage V3 and an output terminal coupled to the inverting input terminal, wherein the voltage V3 is a voltage in proportion to a difference between the input voltage V_(IN) and the output voltage V_(OUT), i.e. V3=K3×K1×(V_(IN)−V_(OUT)). The transistor M2 is coupled between the output terminal of the amplifier 410 and a node N₃, and the transistor M3 is coupled between the node N₃ and the current source 420, wherein the ramp signal S_(RAMP) is a voltage at the node N₃. The transistors M2 and M3 are controlled by the PWM signal S_(PWM) and a signal SB_(PWM), respectively, wherein the signal SB_(PWM) is a reversed signal for the PWM signal S_(PWM). Therefore, the transistor M2 is turned on and the transistor M3 is turned off when an active state of the PWM signal S_(PWM) is asserted, and the transistor M2 is turned off and the transistor M3 is turned on when an inactive state of the PWM signal S_(PWM) is asserted.

FIG. 4B shows a waveform diagram of the signals in the ramp generator 400 of FIG. 4A. Referring to FIG. 4A and FIG. 4B together, the ramp signal S_(RAMP) represents the voltage of the node N₃, i.e. a voltage across the capacitor C_(OFF). When an active state of the PWM signal S_(PWM) is asserted, the transistor M2 is turned on and the transistor M3 is turned off, such that the capacitor C_(OFF) is charged by the amplifier 410 via the transistor M2, and then the voltage across the capacitor C_(OFF) is charged to a voltage level of the voltage V3. On the contrary, when an inactive state of the PWM signal S_(PWM) is asserted, the transistor M2 is turned off and the transistor M3 is turned on, such that the capacitor C_(OFF) is discharged by the current source 420 via the transistor M3, and then the voltage across the capacitor C_(OFF) is decreased until a subsequent active state of the PWM signal S_(PWM) is asserted. In the embodiment, the current source 420 may sink a current I3 from the node N₃ to the ground GND to decrease the ramp signal S_(RAMP), wherein the current I3 corresponds to the input voltage V_(IN). The current source 420 is used as an example, and does not limit the invention. In one embodiment, a current value of the current I3 is equal to that of the current I1 of the PWM generator 300 in FIG. 3A, ex. I3=K1×V_(IN)/R_(RT). In another embodiment, the current I3 is a current in proportion to the current I1. Therefore, a voltage variation dV_(RAMP) of the ramp signal S_(RAMP) during the period T_(off) may be given by the following Equation (13):

$\begin{matrix} \begin{matrix} {{dV}_{RAMP} = {\frac{I\; 3}{C_{OFF}}{dT}}} \\ {= {\frac{K\; 1 \times V_{IN}}{R_{RT} \times C_{OFF}}T_{off}}} \\ {= {\frac{K\; 1 \times V_{IN}}{R_{RT} \times C_{OFF}} \times R_{RT}{C_{ON}\left( \frac{K\; 2}{K\; 1} \right)}\frac{V_{IN} - V_{OUT}}{V_{IN}}}} \\ {= {K\; 2\left( \frac{C_{ON}}{C_{OFF}} \right)\left( {V_{IN} - V_{OUT}} \right)}} \\ {{= {{V\; 3} - V_{steady}}},} \end{matrix} & (13) \end{matrix}$

wherein a voltage level V_(steady) represents an ideal steady voltage level of the error signal V_(ERR) in FIG. 1. Therefore, according to the Equation (13), the voltage level V_(steady) of the error signal V_(ERR) may be given by the following Equation (14):

$\begin{matrix} \begin{matrix} {V_{steady} = {{V\; 3} - {K\; 2\left( \frac{C_{ON}}{C_{OFF}} \right)\left( {V_{IN} - V_{OUT}} \right)}}} \\ {= {{K\; 3 \times K\; 1\left( {V_{IN} - V_{OUT}} \right)} - {K\; 2\left( \frac{C_{ON}}{C_{OFF}} \right)\left( {V_{IN} - V_{OUT}} \right)}}} \\ {= {\left( {\left( {K\; 3 \times K\; 1} \right) - {K\; 2\left( \frac{C_{ON}}{C_{OFF}} \right)}} \right){\left( {V_{IN} - V_{OUT}} \right).}}} \end{matrix} & (14) \end{matrix}$

By choosing the parameters K1, K2 and K3 and the capacitors C_(ON) and C_(OFF) appropriately, the error signal V_(ERR) is designed to operate at a direct current (DC) operation voltage level, i.e. the ideal steady voltage level V_(steady).

Referring to FIG. 1, a fine adjustment of the error signal V_(ERR) is automatically performed for a feedback loop of the DC-DC converter 100 according to the determined DC operation voltage level of the error signal V_(ERR), so as to determine a time period that the trigger signal S_(TR) is triggered for every period T of the PWM signal S_(PWM), thus obtaining a pseudo fix frequency PWM controller.

FIG. 5 shows an example illustrating a waveform diagram of the signals of the DC-DC converter 100 of FIG. 1. By using the error amplifier 160 to generate the error signal V_(ERR) and comparing the error signal V_(ERR) with the ramp signal S_(RAMP) to adjust a duty cycle of the PWM signal S_(PWM), an included angle θ between the error signal V_(ERR) and the ramp signal S_(RAMP) is large at time t4 and sufficient to avoid noise interference, thus increasing a signal to noise ratio (SNR) thereof. FIG. 6 shows another example illustrating a waveform diagram of the signals of the DC-DC converter 100 of FIG. 1. Referring to FIG. 6 and FIG. 1 together, the period T_(H) represents that the load 180 has a higher loading, and the period T_(L) represents that the load 180 has a lower loading. In addition, the load 180 is changed from the lower to higher loading at time t5 and changed from the higher to lower loading at time t6. When the loading of the load 180 is changed, the comparator 170 may immediately adjust the time period that the trigger signal S_(TR) is triggered by comparing the ramp signal S_(RAMP) and the error signal V_(ERR). Therefore, the DC-DC converter 100 may promptly provide the output voltage V_(OUT) in response to the loading of the load 180, thereby increasing system stability.

FIG. 7 shows a DC-DC converter 700 according to another embodiment of the invention. The DC-DC converter 700 is applied to a capacitor C2 with a smaller or zero ESR. Compared with the PWM controller 120 of FIG. 1, a PWM controller 720 of the DC-DC converter 700 further comprises a sense unit 730 for sensing a current flowing through the inductor L to generate a sense current I_(sense) to a compensation unit 710, wherein the sense current I_(sense) corresponds to the loading of the load 180. The compensation unit 710 comprises a resistor 712, a capacitor 714, a resistor R_(comp) coupled between the error amplifier 160 and the comparator 170, and a current source 716 for sinking a current I4 from the resistor R_(comp) to the ground GND. In one embodiment, the current I4 is a current in proportion to the sense current I_(sense). The current source 716 is used as an example, and does not limit the invention. In the embodiment, a current value of the current I4 is equal to that of the sense current I_(sense). Therefore, a voltage across the resistor R_(comp) is determined according to the sense current I_(sense) and a resistance of the resistor R_(comp). The compensation unit 710 receives the error signal V_(ERR) and generates a compensation signal V_(COMP) to the comparator 170 according to the error signal V_(ERR) and the voltage across the resistor R_(comp), such that the comparator 170 of the PWM controller 720 may compare the compensation signal V_(COMP) with the ramp signal S_(RAMP) provided by the ramp generator 130 to generate the trigger signal S_(TR). The compensation signal V_(COMP) comprises a feedback signal from the output voltage V_(OUT) associated with a feedback signal from the current flowing through the inductor L, thus avoiding harmonic oscillation and assuring that the output voltage V_(OUT) is stabilized when the capacitor C2 with a smaller ESR. In addition, by adjusting the resistor R_(comp) or detecting a gain of the sense current I_(sense), a gain of a current loop component is adjusted to increase system stability.

FIG. 8 shows a DC-DC converter 800 according to another embodiment of the invention. Compared with the DC-DC converter 700 of FIG. 7, the sense unit 730 of the DC-DC converter 800 is coupled to a node between the transistor MU and the transistor ML, and senses a current flowing through the transistor ML to generate the sense current I_(sense). Similarly, the sense current I_(sense) provided by the sense unit 830 corresponds to the loading of the load 180.

FIG. 9 shows a DC-DC converter 900 according to another embodiment of the invention. Compared with the DC-DC converter 700 of FIG. 7, the DC-DC converter 900 further comprises a resistor R_(sense) coupled between the transistor ML and the ground GND. Furthermore, the sense unit 730 of the DC-DC converter 900 is coupled to the resistor R_(sense), and senses a current flowing through the resistor R_(sense) to generate the sense current I_(sense). Similarly, the sense current I_(sense) provided by the sense unit 930 corresponds to the loading of the load 180.

FIG. 10 shows a DC-DC converter 1000 according to another embodiment of the invention. In a PWM controller 1020 of the DC-DC converter 1000, the comparator 170 compares the error signal V_(ERR) with a compensation signal V_(comp) provided by a compensation unit 1010 to generate the trigger signal S_(TR). In the embodiment, the sense unit 730 senses a current flowing through the inductor L to generate the sense current I_(sense), wherein the sense current I_(sense) corresponds to the loading of the load 180. In one embodiment, the sense unit 730 may sense a current flowing through the transistor ML to generate the sense current I_(sense). In another embodiment, the DC-DC converter 1000 further comprises a resistor coupled between the transistor ML and the ground GND, e.g. the resistor R_(sense) of FIG. 9, and the sense unit 730 may sense a current flowing through the resistor to generate the sense current I_(sense). The compensation unit 1010 comprises the resistor 712, the capacitor 714, a resistor R_(comp) coupled between the sense unit 730 and the ramp generator 130, and a current source 716 for sinking a current I4 from the resistor R_(comp) to the ground GND. Therefore, the compensation unit 1010 generates the compensation signal V_(COMP) to the comparator 170 according to the sense current I_(sense), a voltage across the resistor R_(comp) and the ramp signal S_(RAMP). Similarly, the compensation signal V_(COMP) comprises a feedback signal from the output voltage V_(OUT) associated with a feedback signal from the current flowing through the inductor L, thus avoiding harmonic oscillation and assuring that the output voltage V_(OUT) is stabilized when the capacitor C2 with a smaller ESR. In addition, by adjusting the resistor R_(comp) or detecting a gain of the sense current I_(sense), a gain of a current loop component is adjusted to increase system stability.

While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. Those who are skilled in this technology can still make various alterations and modifications without departing from the scope and spirit of this invention. Therefore, the scope of the present invention shall be defined and protected by the following claims and their equivalents. 

1. A DC-DC converter for converting an input voltage into an output voltage, comprising: an input node, receiving the input voltage; an output node, providing the output voltage to a load; an inductor coupled between the output node and a first node; a first transistor coupled between the input node and the first node; a second transistor coupled between the first node and a ground; and a Pulse Width Modulation (PWM) controller, comprising: an error amplifier, receiving a reference voltage and the output voltage to generate an error signal according to a difference between the reference voltage and the output voltage; a first comparator, comparing the error signal with a ramp signal to generate a trigger signal; a PWM generator, generating a PWM signal with a fixed turn-on time, wherein a frequency of the PWM signal is adjusted according to the trigger signal, the input voltage and the output voltage; and a ramp generator, generating the ramp signal according to the PWM signal, the input voltage and the output voltage, wherein the PWM controller provides the PWM signal to control the first transistor and the second transistor, so as to convert the input voltage into the output voltage.
 2. The DC-DC converter as claimed in claim 1, further comprising: a control unit for controlling the first transistor and the second transistor according to the PWM signal, wherein the first transistor is turned on and the second transistor is turned off by the control unit when an active state of the PWM signal is asserted, and the first transistor is turned off and the second transistor is turned on by the control unit when an inactive state of the PWM signal is asserted.
 3. The DC-DC converter as claimed in claim 1, wherein the PWM controller further comprises a compensation unit for compensating the error signal.
 4. The DC-DC converter as claimed in claim 1, wherein the PWM generator comprises: a first amplifier having an inverting input terminal coupled to a second node, a non-inverting input terminal for receiving a first voltage corresponding to the input voltage, and an output terminal; a resistor coupled between the second node and the ground; a first current source, providing a first current when the trigger signal is triggered; a third transistor coupled between the second node and the first current source, having a gate coupled to the output terminal of the first amplifier, wherein the first current from the first current source flows to the resistor through the third transistor when the trigger signal is triggered, and the first current corresponds to the first voltage; a second current source, providing a second current according to the first current; a first capacitor coupled between the second current source and the ground, wherein the first capacitor is charged by the second current when the trigger signal is triggered; and a second comparator, comparing a voltage of the first capacitor with a second voltage corresponding to the output voltage to generate the PWM signal.
 5. The DC-DC converter as claimed in claim 1, wherein the ramp generator comprises: a third node for providing the ramp signal; a second amplifier having a non-inverting input terminal for receiving a third voltage corresponding to a difference between the input voltage and the output voltage, an inverting input terminal, and an output terminal coupled to the inverting input terminal; a first switch coupled between the output terminal of the second amplifier and the third node, wherein the first switch is controlled by the PWM signal; a second capacitor coupled between the third node and the ground; a third current source coupled to the ground; and a second switch coupled between the third current and the third node, wherein the second switch is controlled by the PWM signal.
 6. The DC-DC converter as claimed in claim 5, wherein the PWM signal controls the first switch to turn on and controls the second switch to turn off when an active state of the PWM signal is asserted, such that a voltage level of the ramp signal is equal to that of the third voltage.
 7. The DC-DC converter as claimed in claim 5, wherein the PWM signal controls the first switch to turn off and controls the second switch to turn on when an inactive state of the PWM signal is asserted, such that the third current source sinks a third current corresponding to the input voltage from the third node to the ground to decrease a voltage level of the ramp signal.
 8. A DC-DC converter for converting an input voltage into an output voltage, comprising: an input node, receiving the input voltage; an output node, providing the output voltage to a load; an inductor coupled between the output node and a first node; a first transistor coupled between the input node and the first node; a second transistor coupled between the first node and a ground; and a Pulse Width Modulation (PWM) controller, comprising: an error amplifier, receiving a reference voltage and the output voltage to generate an error signal according to a difference between the reference voltage and the output voltage; a sense unit, generating a sense current corresponding to a loading of the load; a compensation unit, generating a compensation signal according to the error signal and the sense current; a first comparator, comparing the compensation signal with a ramp signal to generate a trigger signal; a PWM generator, generating a PWM signal with a fixed turn-on time, wherein a frequency of the PWM signal is adjusted according to the trigger signal, the input voltage and the output voltage; and a ramp generator, generating the ramp signal according to the PWM signal, the input voltage and the output voltage, wherein the PWM controller provides the PWM signal to control the first transistor and the second transistor, so as to convert the input voltage into the output voltage.
 9. The DC-DC converter as claimed in claim 8, further comprising: a control unit for controlling the first transistor and the second transistor according to the PWM signal, wherein the first transistor is turned on and the second transistor is turned off by the control unit when an active state of the PWM signal is asserted, and the first transistor is turned off and the second transistor is turned on by the control unit when an inactive state of the PWM signal is asserted.
 10. The DC-DC converter as claimed in claim 8, wherein the PWM generator comprises: a first amplifier having an inverting input terminal coupled to a second node, a non-inverting input terminal for receiving a first voltage corresponding to the input voltage, and an output terminal; a resistor coupled between the second node and the ground; a first current source, providing a first current when the trigger signal is triggered; a third transistor coupled between the second node and the first current source, having a gate coupled to the output terminal of the first amplifier, wherein the first current from the first current source flows to the resistor through the third transistor when the trigger signal is triggered, and the first current corresponds to the first voltage; a second current source, providing a second current according to the first current; a first capacitor coupled between the second current source and the ground, wherein the first capacitor is charged by the second current when the trigger signal is triggered; and a second comparator, comparing a voltage of the first capacitor with a second voltage corresponding to the output voltage to generate the PWM signal.
 11. The DC-DC converter as claimed in claim 8, wherein the ramp generator comprises: a third node for providing the ramp signal; a second amplifier having a non-inverting input terminal for receiving a third voltage corresponding to a difference between the input voltage and the output voltage, an inverting input terminal, and an output terminal coupled to the inverting input terminal; a first switch coupled between the output terminal of the second amplifier and the third node, wherein the first switch is controlled by the PWM signal; a second capacitor coupled between the third node and the ground; a third current source coupled to the ground; and a second switch coupled between the third current and the third node, wherein the second switch is controlled by the PWM signal.
 12. The DC-DC converter as claimed in claim 11, wherein the PWM signal controls the first switch to turn on and controls the second switch to turn off when an active state of the PWM signal is asserted, such that a voltage level of the ramp signal is equal to that of the third voltage.
 13. The DC-DC converter a claimed in claim 11, wherein the PWM signal controls the first switch to turn off and controls the second switch to turn on when an inactive state of the PWM signal is asserted, such that the third current source sinks a third current corresponding to the input voltage from the third node to the ground to decrease a voltage level of the ramp signal.
 14. The DC-DC converter as claimed in claim 8, wherein the compensation unit comprises: a resistor coupled between the error amplifier and the first comparator, wherein a voltage across the resistor corresponds to the sense current, and the compensation signal is generated according to the error signal and the voltage across the resistor.
 15. A DC-DC converter for converting an input voltage into an output voltage, comprising: an input node, receiving the input voltage; an output node, providing the output voltage to a load; an inductor coupled between the output node and a first node; a first transistor coupled between the input node and the first node; a second transistor coupled between the first node and a ground; and a Pulse Width Modulation (PWM) controller, comprising: an error amplifier, receiving a reference voltage and the output voltage to generate an error signal according to a difference between the reference voltage and the output voltage; a sense unit, generating a sense current corresponding to a loading of the load; a compensation unit, generating a compensation signal according to the sense current and a ramp signal; a first comparator, comparing the compensation signal with error signal to generate a trigger signal; a PWM generator, generating a PWM signal with a fixed turn-on time, wherein a frequency of the PWM signal is adjusted according to the trigger signal, the input voltage and the output voltage; and a ramp generator, generating the ramp signal according to the PWM signal, the input voltage and the output voltage, wherein the PWM controller provides the PWM signal to control the first transistor and the second transistor, so as to convert the input voltage into the output voltage.
 16. The DC-DC converter as claimed in claim 15, further comprising: a control unit for controlling the first transistor and the second transistor according to the PWM signal, wherein the first transistor is turned on and the second transistor is turned off by the control unit when an active state of the PWM signal is asserted, and the first transistor is turned off and the second transistor is turned on by the control unit when an inactive state of the PWM signal is asserted.
 17. The DC-DC converter as claimed in claim 15, wherein the PWM generator comprises: a first amplifier having an inverting input terminal coupled to a second node, a non-inverting input terminal for receiving a first voltage corresponding to the input voltage, and an output terminal; a resistor coupled between the second node and the ground; a first current source, providing a first current when the trigger signal is triggered; a third transistor coupled between the second node and the first current source, having a gate coupled to the output terminal of the first amplifier, wherein the first current from the first current source flows to the resistor through the third transistor when the trigger signal is triggered, and the first current corresponds to the first voltage; a second current source, providing a second current according to the first current; a first capacitor coupled between the second current source and the ground, wherein the first capacitor is charged by the second current when the trigger signal is triggered; and a second comparator, comparing a voltage of the first capacitor with a second voltage corresponding to the output voltage to generate the PWM signal.
 18. The DC-DC converter as claimed in claim 15, wherein the ramp generator comprises: a third node for providing the ramp signal; a second amplifier having a non-inverting input terminal for receiving a third voltage corresponding to a difference between the input voltage and the output voltage, an inverting input terminal, and an output terminal coupled to the inverting input terminal; a first switch coupled between the output terminal of the second amplifier and the third node, wherein the first switch is controlled by the PWM signal; a second capacitor coupled between the third node and the ground; a third current source coupled to the ground; and a second switch coupled between the third current and the third node, wherein the second switch is controlled by the PWM signal.
 19. The DC-DC converter as claimed in claim 18, wherein the PWM signal controls the first switch to turn on and controls the second switch to turn off when an active state of the PWM signal is asserted, such that a voltage level of the ramp signal is equal to that of the third voltage.
 20. The DC-DC converter as claimed in claim 18, wherein the PWM signal controls the first switch to turn off and controls the second switch to turn on when an inactive state of the PWM signal is asserted, such that the third current source sinks a third current corresponding to the input voltage from the third node to the ground to decrease a voltage level of the ramp signal.
 21. The DC-DC converter as claimed in claim 15, wherein the compensation unit comprises: a resistor coupled between the sense unit and the ramp generator, wherein a voltage across the resistor corresponds to the sense current, and the compensation signal is generated according to the sense current, the ramp signal and the voltage across the resistor. 